The effect of La2CuO4 sensing electrode thickness on a potentiometric NOx sensor response

In order to further understand the different contributions to NO_x sensing mechanism as well as the importance of electrode geometry, solid state potentiometric sensors with varying La₂CuO₄ sensing electrode thicknesses were studied. These sensors (with a Pt counter electrode) showed a dependence of NO₂ sensitivity which decreased with increasing thickness in the temperature range of 550-650 °C. They also showed NO sensitivity that was independent of thickness at 400 °C and 600 °C, but varied at temperatures between. This behavior was attributed to multiple mechanistic contributions explained by Differential Electrode Equilibria.     

Improvement of the NOx selectivity for a planar YSZ sensor

In recent years planar yttria-stabilized zirconia (YSZ) based electrochemical gas sensors for automotive exhaust applications have become a major source of interest. The present work aims to develop a sensor for industrialisation. For this reason planar YSZ-based electrochemical sensors using two metallic electrodes (platinum and gold) were fabricated using screen-printing technology and tested in a laboratory test bench for different concentrations of pollutant gas such as CO, NO, NO₂ and hydrocarbons in oxygen rich atmosphere. It was furthermore shown that the selectivity towards NO_x could be highly reinforced by deposing a catalytic filter consisting of 1.7-4.5 wt.% Pt dispersed on alumina directly on the sensing elements. This filter was characterized by the use of SEM, TPD and XRD.     

Diesel engine dynamometer testing of impedancemetric NOx sensors

Prototype solid-state electrochemical sensors using a dense gold sensing electrode, porous yttria-stabilized zirconia (YSZ) electrolyte, and a platinum counter electrode (Au/YSZ/Pt) were evaluated for measuring NO_x (NO and NO₂) in diesel exhaust. Both electrodes were exposed to the test gas (i.e., there was no reference gas for the counter electrode). An impedancemetric method was used for NO_x measurements, where the phase angle was used as the response signal. A portion of the tailpipe exhaust from the dynamometer test stand was extracted and fed into a furnace containing the experimental sensor. The prototype sensor was tested along with a commercially available NO_x sensor. Simultaneous measurements for NO_x, O₂, CO₂, H₂O, CO, and CH₄ in a separate feed stream were made using Fourier transform infrared (FTIR) spectroscopy and an oxygen paramagnetic analyzer. The experimental sensor showed very good measurement capability for NO in the range of 25-250 ppm, with a response paralleling that of the FTIR and commercial sensor. The prototype sensor showed better sensitivity to NO_x at the lower concentration ranges. O₂ is an interferent for the experimental sensor, resulting in decreased sensitivity for measurement of NO_x. Methods to overcome this interference are discussed.     

Solid-state potentiometric SO2 sensor combining NASICON with V2O5-doped TiO2 electrode

A compact tubular sensor based on NASICON (sodium super ionic conductor) and V₂O₅-doped TiO₂ sensing electrode was designed for the detection of SO₂. In order to reduce the size of the sensor, a thick-film of NASICON was formed on the outer surface of a small Al₂O₃ tube; furthermore, a thin layer of V₂O₅-doped TiO₂ with nanometer size was attached on the NASICON as a sensing electrode. This paper investigated the influence of V₂O₅ doping and sintering temperature on the characteristics of the sensor. The sensor attached with 5 wt% V₂O₅-doped TiO₂ sintered at 600 °C exhibited excellent sensing properties to 1–50 ppm SO₂ in air at 200–400 °C. The EMF value of the sensor was almost proportional to the logarithm of SO₂ concentration and the sensitivity (slope) was −78 mV/decade at 300 °C. It was also seen that the sensor showed a good selectivity to SO₂ against NO, NO₂, CH₄, CO, NH₃ and CO₂. Moreover, the sensor had speedy response kinetics to SO₂ too, the 90% response time to 50 ppm SO₂ was 10 s, and the recovery time was 35 s. On the basis of XPS analysis for the SO₂-adsorbed sensing electrode, a sensing mechanism involving the mixed potential at the sensing electrode was proposed.     

Design and experimental validation of an extended Kalman filter-based NOx concentration estimator in selective catalytic reduction system applications

This paper presents an extended Kalman filter (EKF) based approach of integrating NO_x and NH₃ sensors to estimate the NO_x concentrations in Diesel engine selective catalytic reduction (SCR) aftertreatment systems. NO_x sensors have been commonly used by vehicles for aftertreatment system control and onboard diagnostics (OBD) purposes. However, most currently available NO_x sensors are cross-sensitive to ammonia. Based on the experimental observations and physical inferences, the cross-sensitivity characteristics may change with temperature and is hard to be predicted by a model. This feature limits the applications of NO_x sensors on urea-SCR systems where ammonia is the reductant for NO_x conversions. Grounded in the insight into SCR dynamics and NO_x sensor properties, a novel approach of using an extended Kalman filter to estimate the actual exhaust gas NO_x concentration was proposed. The estimator was examined by NO_x measurements from a Horiba gas analyzer under different engine operating conditions. The experimental results show that the EKF-based approach can significantly improve the accuracy of NO_x concentration measurements from the original NO_x sensor readings.     

Development of SAW-based multi-gas sensor for simultaneous detection of CO2 and NO2

A 440 MHz wireless and passive surface acoustic wave (SAW)-based multi-gas sensor integrated with a temperature sensor was developed on a 41° YX LiNbO₃ piezoelectric substrate for the simultaneous detection of CO₂, NO₂, and temperature. The developed sensor was composed of a SAW reflective delay lines structured by an interdigital transducer (IDT), ten reflectors, a CO₂ sensitive film (Teflon AF 2400), and a NO₂ sensitive film (indium tin oxide). Teflon AF 2400 was used for the CO₂ sensitive film because it provides a high CO₂ solubility, with good permeability and selectivity. For the NO₂ sensitive film, indium tin oxide (ITO) was used. Coupling of mode (COM) modeling was conducted to determine the optimal device parameters prior to fabrication. Using the parameters determined by the simulation results, the device was fabricated and then wirelessly measured using a network analyzer. The measured reflective coefficient S₁₁ in the time domain showed high signal/noise (S/N) ratio, small signal attenuation, and few spurious peaks. The time positions of the reflection peaks were well matched with the predicted values from the simulation. High sensitivity and selectivity were observed at each target gas testing. The obtained sensitivity was 2.12°/ppm for CO₂ and 51.5°/ppm for NO₂, respectively. With the integrated temperature sensor, temperature compensation was also performed during gas sensitivity evaluation process.     

Room-temperature CO2 sensing using metal-insulator-semiconductor capacitors comprising atomic-layer-deposited La2O3 thin films

Room temperature detection of CO₂ using metal-insulator-silicon (MIS) devices is reported. These devices comprise atomic layer deposited La₂O₃ thin films as the gas-sensitive dielectric layer and Pt, Pt/Ta and Al as the electrodes. Physical mechanisms that lead to the detection of CO₂ at room temperature are discussed.     

Au-doped WO3-based sensor for NO2 detection at low operating temperature

Pure and Au-doped WO₃ powders for NO₂ gas detection were prepared by a colloidal chemical method, and characterized via X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The NO₂ sensing properties of the sensors based on pure and Au-doped WO₃ powders were investigated by HW-30A gas sensing measurement. The results showed that the gas sensing properties of the doped WO₃ sensors were superior to those of the undoped one. Especially, the 1.0 wt% Au-doped WO₃ sensor possessed larger response, better selectivity, faster response/recovery and better longer term stability to NO₂ than the others at relatively low operating temperature (150 °C).     

Ultrahigh ethanol response of SnO2 nanorods at low working temperature arising from La2O3 loading

The influences of La₂O₃ loading on the ethanol sensing properties of SnO₂ nanorods were investigated. An obvious enhancement of response was obtained. The response of 5 wt% La₂O₃ loaded SnO₂ nanorods was up to 213 for 100 ppm ethanol at low working temperature of 200 °C, while that of pure SnO₂ nanorods is 45.1. The improvement in response might be attributed to the presence of basic sites, which facilitated the dehydrogenation process. While the working temperature was increased to 300 °C, the sensor response decreased to 16 for 100 ppm ethanol. Additionally, the La₂O₃ loaded SnO₂ nanorods sensors showed good selectivity to ethanol over methane and hydrogen. Our results demonstrated that the La₂O₃ loaded SnO₂ nanorods were promising in fabricating high performance ethanol sensors which could work at low temperature.     

In2O3 + xBaO (x = 0.5-5 at.%) - A novel material for trace level detection of NOx in the ambient

Indium oxide (In₂O₃) doped with 0.5-5 at.% of Ba was examined for their response towards trace levels of NO_x in the ambient. Crystallographic phase studies, electrical conductivity and sensor studies for NO_x with cross interference for hydrogen, petroleum gas (PG) and ammonia were carried out. Bulk compositions with x ≤ 1 at.% of Ba exhibited high response towards NO_x with extremely low cross interference for hydrogen, PG and ammonia, offering high selectivity. Thin films of 0.5 at.% Ba doped In₂O₃ were deposited using pulsed laser deposition technique using an excimer laser (KrF) operating at a wavelength of (λ) 248 nm with a fluence of ∼3 J/cm² and pulsed at 10 Hz. Thin film sensors exhibited better response towards 3 ppm NO_x quite reliably and reproducibly and offer the potential to develop NO_x sensors (Threshold limit value of NO₂ and NO is 3 and 25 ppm, respectively).     

Necked ZnO nanoparticle-based NO2 sensors with high and fast response

The NO₂ gas sensing characteristics of semiconductor type gas sensors with channels composed of necked ZnO nanoparticles (NPs) were investigated in this study. The heat treatment of the NPs at 400 °C led to their necking and coarsening. The response of the necked-NP-based sensors was as high as 100 when exposed to 0.2 ppm of NO₂ at 200 °C. As the concentration of NO₂ increased to 5 ppm, their response was enhanced to approximately 400. During the repeated injection of NO₂ gas with a concentration of 0.4 ppm, the sensors exhibited stable response characteristics. Furthermore, the 90% response and recovery times of the gas sensor were as fast as 13 and 10 s, respectively. These observations indicate that the non-agglomerated necking of the NPs induced by the heat treatment significantly enhances the gas sensing characteristics of the NP-based gas sensors.     

A selective room temperature formaldehyde gas sensor using TiO2 nanotube arrays

A new gas sensor using TiO₂ nanotube arrays was fabricated and explored for formaldehyde detection at room temperature. Highly ordered vertically grown TiO₂ nanotube arrays were synthesized by using the conventional electrochemical anodization process. The sensor using the fabricated nanotube arrays as the sensing elements demonstrated a good response to different concentrations of formaldehyde from 10 to 50 ppm and a very good selectivity over other reducing gas species such as ethanol and ammonia at room temperature. While the exact sensing mechanism is unclear, some possibilities are briefly discussed.     

Ab initio calculations of NO2 and SO2 chemisorption onto non-polar ZnO surfaces

The authors present an ab initio study of NO₂ and SO₂ chemisorption onto non-polar ZnO and ZnO surfaces with the aim of providing theoretical hints for further developments in gas sensors. From first principles calculations (DFT-GGA approximation), the most relevant surface reduction scenarios are analyzed and, subsequently, considered in the chemisorption study. First, calculations indicate that NO₂ adsorbs avidly onto Zn surface atoms. This is compatible with the oxidizing character of NO₂. Second, results also explain the sensor poisoning by SO₂ adsorption (since this molecule competes with NO₂ for the same adsorption sites) and indicate that poisoning can only be reverted at typical operation temperatures (T ≤ 700 °C) in the case of stoichiometric ZnO surfaces.     

Nanostructured spinel ZnCo2O4 for the detection of LPG

Nanostrucutred spinel ZnCo₂O₄ (∼26-30 nm) was synthesized by calcining the mixed precursor (consisting of cobalt hydroxyl carbonate and zinc hydroxyl carbonate) in air at 600 °C for 5 h. The mixed precursor was prepared through a low cost and simple co-precipitation/digestion method. The transformation of the mixed precursor into nanostructured spinel ZnCo₂O₄ upon calcinations was confirmed by X-ray diffraction (XRD) measurement, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM). To demonstrate the potential applicability of ZnCo₂O₄ spinel in the fabrication of gas sensors, its LPG sensing characteristics were systematically investigated. The ZnCo₂O₄ spinel exhibited outstanding gas sensing characteristics such as, higher gas response (∼72-50 ppm LPG gas at 350 °C), response time (∼85-90 s), recovery time (∼75-80 s), excellent repeatability, good selectivity and relatively lower operating temperature (∼350 °C). The experimental results demonstrated that the nanostructured spinel ZnCo₂O₄ is a very promising material for the fabrication of LPG sensors with good sensing characteristics. Plausible LPG sensing mechanism is also discussed.     

NO2 gas sensing properties of amorphous InGaZnO4 submicron-tubes prepared by polymeric fiber templating route

One-dimensional (1D) amorphous InGaZnO₄ (a-IGZO) submicron-tubes were synthesized in a method involving an electrospun polymeric fiber template and the direct RF-sputter-coating of a-IGZO films combined with subsequent calcination at 450 °C. The a-IGZO hollow fibers with a diameter of 300 nm and a shell thickness of 20–30 nm showed an amorphous structure, as confirmed by XRD and HR-TEM analyses. Gas sensors using semiconducting a-IGZO tube networks exhibited n-type gas sensing characteristics and a 3.7-fold higher gas response (R_gas/R_air = 109.5 at 2 ppm NO₂) compared to (R_gas/R_air = 29.4) planar a-IGZO thin films at an operating temperature of 300 °C. The enhanced gas response of a-IGZO tubes is attributed to the greater space charge modulation depth associated with the thin shell structures and the porous networks which are readily accessible by gas.     

Glucose-mediated hydrothermal synthesis and gas sensing characteristics of WO3 hollow microspheres

Tungsten-coated carbon microspheres were prepared by one-pot hydrothermal reaction of an aqueous solution containing glucose and sodium tungstate. The spheres were converted into WO₃ hollow microspheres by the decomposition of their core carbon. The [glucose]/[sodium tungstate] ratio of the stock solution determined not only the morphology of the precursors but also the phase of the powders after calcination. The WO₃ hollow microspheres showed a higher gas response and more selective detection of 0.5–2.5 ppm NO₂ than WO₃ solid and nano-porous microspheres did. The enhanced NO₂ sensing characteristics are explained in relation to the surface area, pore volume, and hollow morphology.     

Mixed-potential-type zirconia-based NO2 sensor with high-performance three-phase boundary

This paper focuses on the gas sensing properties of the mixed-potential-type NO₂ sensor based on yttria stabilized zirconia (YSZ) and NiO electrode. The sensing performance of the sensor was improved by modifying the three-phase boundary (TPB). Hydrofluoric acid with different concentrations (10%, 20% and 40%) was used to corrode YSZ substrate to obtain large superficial area of TPB. The scanning electron microscope and atomic force microscopic images showed that the 40% HF could form the largest superficial area at the same corroding time (3 h). The sensitivity of the sensor using the YSZ plate corroded with 40% hydrofluoric acid to 20-500 ppm NO₂ was 76 mV/decade at 850 °C, which was the largest among the examined HF concentrations. It was also seen that the sensor showed a good selectivity and speedy response kinetics to NO₂. On the basis of the measurements of anodic and cathodic polarization curves, as well as the complex impedance of the device, the sensing mechanism was confirmed to involve a mixed potential at the oxide sensing electrode.     

Synthesis of novel SnO2/ZnSnO3 core-shell microspheres and their gas sensing properities

Large-scale novel core-shell structural SnO₂/ZnSnO₃ microspheres were successfully synthesized by a simple hydrothermal method with the help of the surfactant poly(vinyl pyrrolidone) PVP. The as-synthesized samples were characterized using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). The results indicate that the shell was formed by single crystalline ZnSnO₃ nanorods and the core was formed by aggregated SnO₂ nanoparticles. The effects of PVP and hydrothermal time on the morphology of SnO₂/ZnSnO₃ were investigated. A possible formation mechanism of these hierarchical structures was discussed. Moreover, the sensor performance of the prepared core-shell SnO₂/ZnSnO₃ nanostructures to ethanol was studied. The results indicate that the as-synthesized samples exhibited high response and quick response-recovery to ethanol.     

Acoustic wave-based NO2 sensor: Ink-jet printed active layer

Microsphere-templated BaCO₃ films with well-defined area were deposited onto quartz crystal microbalances by thermal ink-jet printing, and the devices were characterized with respect to their microstructures and NO₂ sensing characteristics. Highly porous three-dimensional BaCO₃ frameworks with promising sensor characteristics were obtained. The printed thin films exhibited reversible frequency shifts following exposure to NO₂ and subsequent recovery under CO/CO₂ at 400 °C. The feasibility of controlled deposition of complex functional films in controlled patterns is discussed in the context of the direct-write features of ink-jet printing.     

SnO2-based R134a gas sensor: Sensing materials preparation, gas response and sensing mechanism

Undoped SnO₂ and porous Al₂O₃ powders were obtained through a simple chemical precipitation process. SnO₂-based gas sensing materials and Al₂O₃ catalytic coating loaded with a noble metal were prepared by impregnation. The SnO₂ and Al₂O₃ powders were characterized by TEM, SEM, nitrogen adsorption-desorption experiment, FT-IR and in situ XRD. Gas responses of the SnO₂-based gas sensors were measured in a static state. The experimental results indicated that the response towards R134a of the SnO₂-based gas sensor can be significantly enhanced by loading noble metal and using catalytic coating. The sensor based on a double layer film SnO₂ (Au)/Al₂O₃ (Au) showed satisfactory results including large response, good selectivity, high long-term stability, fast response and recovery, revealing its potential application in the detection of refrigerants and the maintenance of air condition systems. Finally, a gas sensing mechanism for R134a is suggested and proved by bond energy data, FT-IR spectrum and in situ XRD.